1. Field of the Invention
2. Description of the Prior Art
The present invention relates to optical fiber transmission and more specifically to compensating chromatic dispersion and chromatic dispersion slope in optical fiber transmission systems.
The index profile of optical fibers is generally described by the shape of the graph of the function that associates the refractive index of the fiber with its radius. It is conventional to plot the distance r from the center of the fiber on the abscissa axis and the difference between the refractive index and the refractive index of the cladding of the fiber on the ordinate axis. The expressions xe2x80x9cstepxe2x80x9d index profile, xe2x80x9ctrapeziumxe2x80x9d index profile and xe2x80x9ctrianglexe2x80x9d index profile are therefore used with reference to graphs that are respectively step-shaped, trapezium-shaped and triangular. These curves are generally representative of the theoretical or set point profile of the fiber and fiber fabrication constraints can yield a significantly different profile.
It is advantageous to manage chromatic dispersion in new high bit rate wavelength division multiplexed transmission networks, especially for bit rates greater than or equal to 40 Gbit/s or 160 Gbit/s; the objective, in order to limit pulse widening, is to obtain substantially zero cumulative chromatic dispersion over the link, for all wavelengths of the multiplex. A cumulative dispersion value of a few tens of ps/nm is generally acceptable. It is also beneficial to avoid, in the vicinity of wavelengths used in the system, zero values of the local chromatic dispersion, for which the non-linear effects are strongest. Furthermore, it is also beneficial to limit the cumulative chromatic dispersion slope over the range of the multiplex to prevent or limit distortion between multiplex channels. The chromatic dispersion slope is conventionally the derivative of chromatic dispersion with respect to wavelength.
Step index fibers, also known as single mode fibers (SMF), are conventionally used as line fibers in optical fiber transmission systems. The applicant""s ASMF 200 step index monomode fiber has a chromatic dispersion cancellation wavelength xcex0 from 1 300 to 1 320 nm and a chromatic dispersion less than or equal to 3.5 ps/(nm.km) in a range from 1 285 to 1 330 nm and of the order of 17 ps/(nm.km) at 1 550 nm. The chromatic dispersion slope at 1 550 nm is of the order of 0.06 ps/(nm2.km).
Dispersion shifted fibers (DSF) have also become available. At the transmission wavelength at which they are used, which is generally different from the wavelength of 1.3 xcexcm for which the dispersion of silica is substantially zero, the chromatic dispersion is substantially zero; in other words, the non-zero chromatic dispersion of the silica is compensatedxe2x80x94whence the use of the term xe2x80x9cshiftedxe2x80x9dxe2x80x94by increasing the index difference xcex94n between the core of the fiber and the optical cladding. That index difference offsets the wavelength at which zero chromatic dispersion is obtained; it is achieved by introducing dopants into the preform, when fabricating the latter, for example by an MCVD process known in the art, and which is not described in more detail here.
Non-zero dispersion shifted fibers (NZxe2x88x92DSF+) are dispersion shifted fibers having a positive non-zero chromatic dispersion at the wavelengths at which they are used, typically around 1 550 nm. At these wavelengths these fibers have a low chromatic dispersion, typically less than 11 ps/(nm.km) and a chromatic dispersion slope from 0.04 to 0.1 ps/(nm2.km) at 1 550 nm.
The document FR-A2 790 107 proposes a line fiber especially suitable for dense wavelength division multiplex transmission with a channel spacing of 100 GHz or less for a bit rate per channel of 10 Gbit/s; at a wavelength of 1 550 nm, this fiber has an effective surface area greater than or equal to 60 xcexcm2, a chromatic dispersion from 6 to 10 ps/(nm.km), and a chromatic dispersion slope less than 0.07 ps/(nm2.km).
French patent application number 00/02316 filed Feb. 24, 2000, whose title in translation is xe2x80x9cAn optical fiber exhibiting monomode behavior in-cable for wavelength division multiplex optical fiber transmission networksxe2x80x9d, proposes a line fiber which has, at a wavelength of 1 550 nm, a chromatic dispersion from 5 to 11 ps/(nm.km), a ratio of chromatic dispersion to chromatic dispersion slope from 250 to 370 nm and a ratio of the square of the effective surface area to the chromatic dispersion slope greater than 8xc3x97104 xcexcm2.nm2.km/ps. This line fiber has a range of use from 1 300 to 1 625 nm. In one example described in the above application, its dispersion is compensated by dispersion compensating fiber having a chromatic dispersion of xe2x88x92100 ps/(nm.km) and a ratio of chromatic dispersion to chromatic dispersion slope of 260 nm.
Using short lengths of dispersion compensating fiber (DCF) to compensate chromatic dispersion and chromatic dispersion slope in SMF or NZxe2x88x92DSF+ used as line fiber is known in the art. One example of a transmission system in which chromatic dispersion in an SMF line fiber is compensated using DCF is described in M. Nishimura et al., xe2x80x9cDispersion compensating fibers and their applicationsxe2x80x9d, OFC""96 Technical Digest ThA1. Such use of dispersion compensating fiber is also mentioned in L. Grxc3xcner-Nielsen et al., xe2x80x9cLarge volume Manufacturing of dispersion compensating fibersxe2x80x9d, OFC""98 Technical Digest TuD5. The drawbacks of this type of fiber are its high cost and its high sensitivity to PMD and to incorporation into a cable.
DCF are also described in various patents. In the vicinity of a wavelength of 1 550 nm they have a negative chromatic dispersion to compensate the cumulative chromatic dispersion in the line fiber, and can also have a negative chromatic dispersion slope to compensate the positive chromatic dispersion slope of the line fiber.
The documents U.S. Pat. Nos. 5,568,583 and 5,361,319 propose a DCF for compensating chromatic dispersion in an SMF which has a dispersion of the order of 17 ps/(nm.km) at 1550 nm.
The document WO-A-99 13366 proposes a dispersion compensating fiber that it is intended to be used in compensation modules to compensate the chromatic dispersion and the chromatic dispersion slope of a Lucent xe2x80x9cTrue Wavexe2x80x9d fiber; the fiber has a chromatic dispersion from 1.5 to 4 ps/(nm.km) and a chromatic dispersion slope of 0.07 ps/(nm2.km). One embodiment of the proposed dispersion compensating fiber has a chromatic dispersion of xe2x88x9227 ps/(nm.km) and a chromatic dispersion slope of xe2x88x921.25 ps/(nm2.km).
EP-A-0 674 193 proposes a dispersion compensating fiber having a chromatic dispersion from xe2x88x9285 to 20 ps/(nm.km); comparative examples of profiles with dispersion values from xe2x88x9220 to 0 ps/(nm.km) are proposed in the figures; the chromatic dispersion slope is positive or very weakly negative for these comparative examples.
U.S. Pat. No. 5,838,867 proposes a dispersion compensating fiber intended for in-line or in-module compensation of chromatic dispersion in a dispersion shifted line fiber.
K. Mukasa et al., xe2x80x9cNovel network fiber to manage dispersion at 1.55 xcexcm with combination of 1.3 xcexcm zero dispersion single mode fiberxe2x80x9d, ECOC 97, Sep. 22-25, 1997, Conference publication No 448, proposes a Reverse Dispersion Fiber (RDF) which has chromatic dispersion and chromatic dispersion slope properties which are the inverse of those of an SMF line fiber. At 1 550 nm the fiber has a chromatic dispersion of xe2x88x9215.6 ps/(nm.km) and a chromatic dispersion slope of xe2x88x920.046 ps/(nm2.km), and thus a ratio of chromatic dispersion to chromatic dispersion slope of the order of 340 nm. It has a W-shaped index profile, with a peak at the center, surrounded by a trench which has a lower index than the cladding. In the above publication the RDF is used as line fiber, alternating with SMF: the cumulative chromatic dispersion and chromatic dispersion slope in an SMF section are compensated by the propagation in the next RDF section. This teaching is also set out in FR-A-2 761 483. The examples of profiles provided in that application are all step index profiles, in most cases with a buried cladding.
The invention proposes a new fiber which can be used to compensate chromatic dispersion in the fiber proposed in application No. 00/02316, previously cited. Compared to the fiber proposed in the Mukasa paper, it has lower attenuation and a larger effective surface area; it can be used more easily as line fiber in a transmission system. Also, the fiber of the invention can be used over a wider band than the fiber proposed in the Mukasa paper.
An optical fiber which has, at a wavelength of 1 550 nm, a chromatic dispersion from xe2x88x9212 ps/(nm.km) to xe2x88x924 ps/(nm.km) and a ratio of chromatic dispersion to chromatic dispersion slope from 250 nm to 370 nm.
The fiber of the invention can advantageously have one or more of the following propagation characteristics:
at a wavelength of 1 550 nm, an effective surface area greater than 20 xcexcm2, and preferably greater than or equal to 30 xcexcm2;
at wavelengths from 1 300 nm to 1 620 nm, and preferably at wavelengths from 1 300 nm to 1 650 nm, bending losses less than or equal to 0.05 dB, and preferably less than or equal to 0.001 dB, after winding 100 turns around a 30 mm radius former;
at wavelengths from 1 300 nm to 1 620 nm, and preferably at wavelengths from 1 300 nm to 1 650 nm, bending losses less than or equal to 100 dB/m, for a bending radius of 10 mm;
at wavelengths from 1 300 nm to 1 620 nm, and preferably at wavelengths from 1 300 nm to 1 650 nm, a sensitivity to microbending less than or equal to 1, and preferably less than or equal to 0.5;
at wavelengths from 1 300 nm to 1 620 nm, and preferably at wavelengths from 1 300 nm to 1 650 nm, an attenuation less than or equal to 0.30 dB/km and preferably less than or equal to 0.25 dB/km;
an in-cable cut-off wavelength less than or equal to 1 400 nm, and preferably less than or equal to 1 300 nm;
a polarization mode dispersion less than or equal to 0.1 ps.kmxe2x88x921/2.
The fiber preferably has an index profile comprising a rectangle or a trapezium with a buried trench and a ring. The index profile can have the following features:
the difference relative to the index of the cladding of the index of the trapezium or rectangle central part of said fiber is from 9.5xc3x9710xe2x88x923 to 11.6xc3x9710xe2x88x923 and the radius of the central part of said fiber whose index is higher than the index of said cladding is from 2.4 xcexcm to 2.9 xcexcm;
the difference relative to the index of the cladding of the index of said buried trench is from xe2x88x926.9xc3x9710xe2x88x923 to xe2x88x923.1xc3x9710xe2x88x923 and the outside radius of said buried trench is from 4.8 xcexcm to 6.9 xcexcm;
the difference relative to the index of the cladding of the index of said ring is from 1.5xc3x9710xe2x88x923 to 8.4xc3x9710xe2x88x923 and the outside radius of said ring is from 7.5 xcexcm to 10.2 xcexcm.
In the case of an index profile which has a trapezium central part, the ratio of the radius of the smaller base of said trapezium to the radius of the larger base of said trapezium is preferably from 0.8 to 1.
The fiber can also have an index profile comprising a trapezium with a buried trench and a ring with ratio of the radius of the smaller base of said trapezium to the radius of the larger base of said trapezium from 0.6 to 0.8. In this case the index profile can have the following features:
the difference relative to the index of the cladding of the index of the trapezium central part of said fiber is from 9.8xc3x9710xe2x88x923 to 11.6xc3x9710xe2x88x923 and the radius of the central part of said fiber whose index is higher than the index of said cladding is from 2.5 xcexcm to 2.9 xcexcm;
the difference relative to the index of the cladding of the index of said buried trench is from xe2x88x927.0xc3x9710xe2x88x923 to xe2x88x923.2xc3x9710xe2x88x923 and the outside radius of said buried trench is from 4.7 xcexcm to 7.0 xcexcm;
the difference relative to the index of the cladding of the index of said ring is from 1.5xc3x9710xe2x88x923 to 8.5xc3x9710xe2x88x923 and the outside radius of said ring is from 7.5 xcexcm to 10.2 xcexcm.
The invention also proposes the use of a fiber of the above kind as dispersion compensating fiber in a wavelength division multiplex optical fiber transmission system. This fiber can be incorporated into a cable and used as line fiber.
The invention finally proposes a wavelength division multiplex optical fiber transmission system including a first section of line fiber and a second section of line fiber consisting of the fiber previously defined.
Advantageously, at a wavelength of 1 550 nm, said line fiber of said first section has a chromatic dispersion from 5 ps/(nm.km) to 11 ps/(nm.km), a chromatic dispersion slope from 0.01 ps/(nm2.km) to 0.04 ps/(nm2.km), an effective surface area from 50 xcexcm2 to 70 xcexcm2 and/or a ratio of chromatic dispersion to chromatic dispersion slope from 250 nm to 370 nm.
The ratio of the length of said first section to the length of said second section is preferably substantially the inverse of the absolute value of the ratio of the chromatic dispersions at a wavelength of 1 550 nm of said fibers of said first and second sections.
In one embodiment of the system, the cumulative chromatic dispersion at a wavelength from 1 450 nm to 1 620 nm, and preferably at a wavelength from 1 450 nm to 1 650 nm, is less than 100 ps/nm, and preferably less than 50 ps/nm, on average over 100 km of transmission.
Other features and advantages of the invention will become apparent on reading the following description of embodiments of the invention, which description is given by way of example and with reference to the accompanying drawings.